1. Field of the Invention
The present invention relates to an air flow meter having a flow meter element, suitable for use in an intake pipe of vehicle internal combustion engine.
2. Description of Related Art
An air flow meter is used for measuring intake air flow amount of an internal combustion engine. The air flow meter is disposed in a bypass passage within an engine intake pipe. The air flow meter has a flow meter element and a heat sensing element, and measures the intake air flow mount base on a value of electric current supplied into the flow meter element.
When the engine operates, the intake air flow amount fluctuate relatively remarkably, so that pulsating flow arises in the intake air flow. The pulsating flow disorders an air flow introduced into the bypass passage, thereby causing a measurement error of the intake air flow amount.
JP-A-8-285659 discloses an air flow meter in which two turbulence reduction grills are provided. The turbulence reduction grills are disposed at an inlet of main air passage and arranged in parallel with each other. Grill directions of these turbulence reduction grills are off-set by 45 degrees to reduce turbulence in the intake air flowing into the main air passage, thereby stabilizing air which flows into the bypass passage.
However, since two turbulence reduction grills are provided at the inlet of the main air flow passage, pressure loss is increased in the main air passage, thereby worsening an engine performance. Further, costs for forming two turbulence reduction grills and for press-inserting or insert-forming the turbulence reduction grills to the inlet of the main air flow passage are increased.
An object of the present invention is to reduce an influence of pulsating flow without increasing a pressure loss by a simple structure, thereby improving measurement accuracy with reducing manufacturing cost and the pressure loss.
According to a first aspect of the present invention, a U-shaped bypass passage is provided in an air passage. The bypass passage is arranged substantially perpendicularly to a primary air flow direction in the air passage and defines an air inlet. A turbulence reduction plate is provided around an air inlet of the bypass passage, and the turbulence reduction plate is arranged substantially in parallel with the primary air flow direction in the air passage, and substantially perpendicularly to an air flow direction in the bypass passage.
Even when a secondary air flow, of which direction is in parallel with an air flow direction in the bypass passage, arises around the air inlet of the bypass passage, the turbulence reduction plate shuts the secondary air flow, thereby reducing an influence of the secondary air flow. Thus, air flow introduced into the bypass passage is stabilized, thereby improving the air flow amount measurement accuracy. Here, the turbulence reduction plate is disposed substantially in parallel with a primary air flow direction in the air passage, so that pressure loss caused by the turbulence reduction plate is enough small, thereby introducing no influence on an engine efficiency. Further, the turbulence reduction plate is simply structured and provided, thereby reducing the manufacturing cost.
According to a second aspect of the present invention, the bypass passage includes two fluid passage next to and in parallel with each other, and a bent portion connecting the two fluid passages. A partition wall partitions the two fluid passages, and a venturi is formed at the air inlet side of the partition wall. The turbulence reduction plate is disposed at a position facing around a center of the venturi.
Thus, the primary air flowing into the venturi is prevented from being disordered while the turbulence reduction plate effectively reduces the influence of the secondary flow, so that stable venturi effect can be attained. The air having passed through the venturi introduces negative pressure acting on an air outlet of the bypass passage, so that the air flow speed in the bypass passage is increased, thereby improving the air flow amount measurement accuracy.